1. Field of the invention
The present invention relates to an optical arrangement for a camera module, a camera module with the optical arrangement and a method of manufacturing the optical arrangement.
2. Description of Related Art
In contrast to a human eye, a camera sensor such as a CMOS (Complementary Metal-Oxide-Semiconductor) or CCD (Charge-coupled Device) image sensor is typically also sensitive to infrared (IR) light. Thus, infrared light components may cause defects in the recorded image due to aberrations in color and brightness.
To eliminate or reduce errors caused by IR light in the displayed image, an IR-cut-off filter is often disposed between the imaging lens and the image sensor, so that IR light is blocked while visible light is transmitted to the image sensor. However, the use of an IR-cut-off filter requires additional space between the sensor and the objective lens and therefore a larger back focal length (BFL). This may increase the thickness of the camera module. Above that, the spatial proximity of the filter to the image sensor requires a good surface finish of the filter, as light ray distortions would be mapped to the nearby image plane and thus may be visible in the recorded images. If using infrared absorbing glasses as infrared filter, another problem arises due to the typically low mechanical stability. Drop tests have shown that a thin infrared absorbing glass filter in the camera module can easily be broken.
US 2014/0043677 A1 discloses an infrared-cut (IR-cut) filter including a substrate and an IR-cut film coated on the substrate. The IR-cut consists of thirty-two high-refractive layers and thirty-two low-refractive layers alternately stacked on the substrate. The first high-refractive layer is in contact with the substrate and the first low-refractive layer is coated on the first high-refractive layer.
US 2013/0286470 A1 discloses an IR-cut filter and a lens module including the IR-cut filter. The IR-cut filter includes a substrate made of sapphire and a film formed on the substrate. The film is configured to increase the IR reflectivity of the substrate and is coated on the substrate, the film including a number of high refractive index layers and a number of low refractive index layers alternately stacked on the substrate.
US 2012/0261550 A1 discloses an optical lens assembly with a filter member for image taking, sequentially arranged from an object side to an image side along an optical axis. The filter member can filter infrared light and comprises a glass that can absorb infrared light.